Aluminium-wettable porous ceramic material

ABSTRACT

A material, for instance used as an aluminium-wettable component ( 21,21′, 41,41′, 51 ), in particular of a cell for the electrowinning of aluminium ( 60 ), comprises an openly porous or reticulated ceramic structure whose surface during use is exposed to and wetted by molten aluminium. The structure is made of ceramic material inert and resistant to molten aluminium, such as alumina, and an aluminium-wettable material that comprises metal oxide and/or partly oxidised metal, in particular of manganese, iron, cobalt, nickel, copper or zinc, which is/are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from the metal oxide and/or partly oxidised metal. The ceramic structure comprises a coating of the aluminium-wettable material on the inert and resistant ceramic material, or is made of a mixture of the inert and resistant material and of the aluminium-wettable ceramic material. The aluminium-wetted component is suitable for use as a cathode ( 21,21 ′), as a sidewall ( 41,41 ′) or as another component ( 51 ) which during use is exposed to molten aluminium ( 60 ) and/or electrolyte ( 5 ), or another oxidising and/or corrosive media at high temperature.

FIELD OF THE INVENTION

[0001] The invention relates to a ceramic material which can be utilisedfor the manufacture of aluminium-wettable and aluminium-wetted ceramiccomponents, in particular for use in aluminium production, for exampleas cathodes, sidewalls and other cell components which during use areexposed to molten aluminium, electrolyte and/or corrosive gases.

BACKGROUND OF THE INVENTION

[0002] Aluminium is produced conventionally by the Hall-Heroult process,by the electrolysis of alumina dissolved in cryolite-based moltenelectrolytes at temperatures up to around 950° C. A Hall-Heroultreduction cell typically has a steel shell provided with an insulatinglining of refractory material, which in turn has a lining of carbonwhich contacts the molten constituents and corrosive gases. Conductorbars connected to the negative pole of a direct current source areembedded in the carbon cathode forming the cell bottom floor. Thecathode is usually an anthracite based carbon lining made of prebakedcathode blocks, joined with a ramming mixture of anthracite, coke andcoal tar, or with glue.

[0003] It has long been recognised that it would be desirable to make(or coat or cover) the cathode of an aluminium electrowinning cell witha refractory boride such as titanium diboride that would render thecathode surface wettable to molten aluminium which in turn would lead toa series of advantages. Many difficulties were encountered in producingrefractory boride coatings which meet up to the rigorous conditions inan aluminium electrowinning cell. Nevertheless, such coatings appliedfrom slurries to carbon bodies have been developed. The most recentslurry-applied coatings are disclosed in WO01/42168 (de Nora/Duruz) andWO01/42531 (Nguyen/Duruz/de Nora).

[0004] U.S. Pat. No. 5,981,081 (Sue) discloses wear and corrosionresistant coatings made of transition metal boride particles dispersedin a matrix of nickel, cobalt or iron. The coatings are applied byexplosion or plasma spraying a mixture of powders of a transition metalboride and a boron containing alloy on a metal substrate and heattreating.

[0005] Previously, it had been proposed to replace the carbon materialof the cathodes of aluminium production cells with ceramic material. Forexample, U.S. Pat. No. 4,560,448 (Sane/Wheeler/Kuivila) discloses aporous component made of aluminium repellent material covered with analuminium-wettable metal boride coating which during use is maintainedby saturating the molten aluminium infiltrating the porous componentwith coating constituents. U.S. Pat. No. 4,650,552 (deNora/Gauger/Fresnel/Adorian/Duruz) discloses an aluminium productioncell component produced from a powder mixture of alumina and aluminium.U.S. Pat. No. 4,600,481 (Sane/Wheeler/Gagescu/Debely/Adorian/Derivaz)discloses a component of an aluminium production cell which is made ofan openly porous matrix, e.g. an alumina matrix, filled with moltenaluminium. The openly porous matrix may comprise an aluminium-wettablecoating made of a boride or nickel. The infiltration of the matrix withaluminium is carried out at a temperature of 10000 to 1500° C.

[0006] Materials made of a ceramic matrix infiltrated with metal havealso been described in the following references. U.S. Pat. No. 4,935,055(Aghajanian/Claar), U.S. Pat. No. 5,194,202 (Yun/Marra/Gurganus/Kelsey)and U.S. Pat. No. 5,676,907 (Ritland/Readey/Stephan/Rulis/Sibold)disclose different methods of infiltrating a ceramic structures, e.g.Al₂O₃, SiN or SiC, with molten aluminium. U.S. Pat. No. 5,043,182(Schultze/Schindler/Deisenroth) discloses a porous Al₂O₃—Al₂TiO₅structure infiltrated under pressure with a molten aluminium alloy.

[0007] U.S. Pat. No. 5,007,475 (Kennedy/Aghajanian) discloses a ceramicstructure, e.g. alumina, infiltrated by molten aluminium with the aid ofan infiltration enhancer consisting of a metal/gas combination selectedfrom Mg/N, Sr/N, Zn/O and Ca/N to which the alumina structure is exposedbefore and during infiltration. It is also contemplated in this patentto use ceramic structures described in U.S. Pat. No. 4,713,360(Newkirk/Dizio) that discloses porous ceramic structures obtained byoxidising aluminium metal with additives selected from Mg, Zn, Si, Na,Li, Ca, B, P, Y, rare earth metals, and possibly non-functional diluentsor impurities, such as Mn, Fe, Cu and W, in an amount of much less than1% of the structure.

OBJECTS OF THE INVENTION

[0008] An object of the invention is to provide an aluminium-wettablecomponent for a cell for the production of aluminium from aluminadissolved in a fluoride-based molten electrolyte.

[0009] Another object of the invention is to provide an aluminium-wettedcomponent which is highly conductive and resistant to molten electrolytefor use as a cathode in a drained cell or in a cell operating with ashallow or deep aluminium pool or as a cell sidewall or anothercomponent which is exposed to molten aluminium, electrolyte and/orcorrosive gases, or as a lining for protecting other cell componentsagainst molten electrolyte, or for making other cell componentsaluminium-wettable.

[0010] A further object of the invention is to provide analuminium-wettable or aluminium-wetted component which can be made fromreadily available materials.

[0011] Yet another object of the invention is to provide analuminium-wettable component which can be wetted with aluminium outsidean aluminium production cell or in-situ by exposure to cathodic moltenaluminium.

[0012] Another object of the invention is to provide an aluminium-wettedcomponent that retains its protective and wettability properties evenwhen exposed to highly oxidising and/or corrosive environments.

[0013] Yet a further object of the invention is to provide aceramic-based or a ceramic-metal material which can be used in anoxidising and/or corrosive media at elevated temperature.

SUMMARY OF THE INVENTION

[0014] A first aspect of the invention relates to an aluminium-wettablecomponent of a cell for the electrowinning of aluminium from aluminadissolved in a fluoride-based molten electrolyte. The componentcomprises an openly porous or reticulated ceramic structure whosesurface during use is exposed to and wetted by molten aluminium. Thestructure is made of a ceramic material inert and resistant to moltenaluminium and an aluminium-wettable material that comprises metal oxideand/or partly oxidised metal which is/are reactable with moltenaluminium to form a surface layer containing alumina, aluminium andmetal derived from the metal oxide and/or partly oxidised metal.

[0015] The inert and resistant ceramic material may comprise at leastone oxide selected from oxides of aluminium, zirconium, tantalum,titanium, silicon, niobium, magnesium and calcium and mixtures thereof,as a simple oxide and/or in a mixed oxide, for example an aluminate ofzinc (ZnAlO₄) or titanium (TiAlO₅). Other suitable inert and resistantceramic materials can be selected amongst nitrides, carbides and boridesand oxycompounds, such as aluminium nitride, AlON, SiAlON, boronnitride, silicon nitride, silicon carbide, aluminium borides, alkaliearth metal zirconates and aluminates, and their mixtures.

[0016] Usually, the reaction of the metal oxide and/or partly oxidisedmetal with molten aluminium involves the reduction of the metal oxideand/or partly oxidised metal and the oxidation of aluminium. For themetal oxide and/or partly oxidised metal to be reducible by moltenaluminium, it is necessary that such a metal be more electronegativethan aluminium. For example, the metal of the metal oxide and/or partlyoxidised metal reducible by molten aluminium is selected from manganese,iron, cobalt, nickel, copper and zinc and combinations thereof.

[0017] The concentration of reactable metal oxide and/or partly oxidisedmetal at the surface of the ceramic structure affects the speed at whichthe structure is wetted by molten aluminium. The surface of the ceramicstructure should contain the reactable metal oxide and/or partlyoxidised metal in an amount of at least 2 to 3 weight %, preferably atleast 5 to 25 weight % of the material making the surface of the ceramicstructure. When the ceramic structure comprises a coating of thealuminium wettable material as described hereafter, the coating maycomprise much more metal oxide and/or partly oxidised metal, e.g. up to50 or even 80 weight % or possibly even more. The electronegativity ofthe metal of the reactable metal oxide and/or partly oxidised metal alsoaffects the speed of aluminium wetting. The fastest wetting of theceramic structure is achieved when the metal of the reactable metaloxide and/or partly oxidised metal is selected from copper, nickel,cobalt, manganese and iron.

[0018] In one embodiment of the invention, the openly porous orreticulated ceramic structure comprises a coating of thealuminium-wettable material on the inert and resistant ceramic material.In other words, the openly porous or reticulated ceramic structureconsists of a skeleton of the inert and resistant ceramic materialcoated with the aluminium-wettable material.

[0019] Such aluminium-wettable coating is usually a slurry-appliedcoating comprising particles of the metal oxide and/or partly oxidisedmetal reactable with molten aluminium in a dried colloidal carrierselected from alumina, ceria, lithia, magnesia, silica, thoria, yttria,zirconia, titanium oxide and zinc oxide, and precursors and mixturesthereof. Further details of such slurry-applied coatings are disclosedin WO01/42168 (de Nora/Duruz), which describes such coatings on solidsubstrates.

[0020] The slurry-applied aluminium-wettable coating may furthercomprise particles of at least one compound selected from metal borides,carbides and nitrides. For example, the aluminium-wettable coatingcomprises the particles of the metal oxide and/or partly oxidised metalreactable with molten aluminium and particles of titanium diboride indried colloidal alumina.

[0021] Particles of the metal boride, carbide or nitride may be coveredwith mixed oxides of metal derived from the dried colloidal carrier andmetal derived from the metal boride, carbide or nitride. To improve thestructure of the coating, the slurry-applied aluminium-wettable coatingcan be obtained from a slurry comprising metal oxide particles thatcombine upon heat treatment with metal derived from the dried colloidalcarrier to form mixed oxides which are miscible with the mixed oxidescovering the particles of the metal boride, carbide or nitride. Suitableslurries producing such a coating are disclosed in WO01/42531(Nguyen/Duruz/de Nora), which describes such coatings on solidsubstrates.

[0022] In another embodiment of the invention, the openly porous ceramicstructure is made of a composition which comprises a mixture of theinert and resistant ceramic material and the aluminium-wettable ceramicmaterial. Such a ceramic structure should comprise a sufficient amountof inert and resistant ceramic material that upon contact/reaction ofthe aluminium-wettable ceramic material with molten aluminium, theoverall ceramic structure retains sufficient mechanical properties.Usually, the aluminium-wettable material makes up less than 15 weight %,usually less than 10 weight %, of the ceramic structure.

[0023] Furthermore, the openly porous ceramic structure may be formed ona reinforcing metal skeleton, in particular a metal mat. Suitable metalsfor such a skeleton include iron and iron alloys and other metals whichare mechanically resistant at elevated temperature.

[0024] For some applications, it may be advantageous to use internalinserts acting as ballast inside a component made of the ceramicstructure, for instance to secure the ceramic structure on the bottom ofan aluminium production cell as disclosed in FIGS. 2 and 3 of U.S. Pat.No. 5,651,874 (de Nora/Sekhar). The internal inserts may be made of ironor iron alloys or other heavy materials. A reinforcing metal can alsoact as ballast.

[0025] The component of the invention has numerous applications some ofwhich are set out hereafter.

[0026] For instance, the component may be a cathode or a cathode lining,for example plate- or wedge-shaped, on a cathode body, in particularmade of carbon material. The component can also be an aluminium poolstabiliser in the form of a plate having a density which is either lowerthan that of molten aluminium so that it can float at the surface of thealuminium pool, or higher than that of molten aluminium so that it canrest at the bottom of the aluminium pool. All of the aforementionedcomponents, which are exposed during use to the product aluminium, canbe placed as such in the cell and wetted during use. Such components maybe top coated with a highly aluminium-wettable start-up layer, forexample as disclosed in WO01/42168 (de Nora/Duruz).

[0027] On the other hand, for certain applications the components mayneed to be wetted with molten aluminium before use. Therefore, thealuminium-wettable component can constitute a skeleton which can beinfiltrated with molten aluminium to form for example a cell sidewall ora sidewall lining, or a wedge-shaped connecting body for joining thesurface of a cell bottom to an adjacent sidewall at the periphery of thecell bottom.

[0028] The invention also relates to an aluminium-wetted component of acell for the electrowinning of aluminium. The aluminium-wetted componentcomprises an openly porous or reticulated ceramic structure which has asurface layer containing alumina, aluminium and another metal, e.g.iron, copper or nickel. Such component is obtainable by exposing tomolten aluminium an openly porous or reticulated aluminium-wettablecomponent made of ceramic material inert and resistant to moltenaluminium, e.g. alumina, and an aluminium-wettable material thatcomprises metal oxide and/or partly oxidised metal, e.g. iron, copper ornickel as oxides and/or partly oxidised metals, which is/are reactablewith molten aluminium as described above.

[0029] The component comprises an openly porous or reticulated ceramicstructure whose surface during use is exposed to and wetted by moltenaluminium. The structure is made of a ceramic material inert andresistant to molten aluminium and an aluminium-wettable material thatcomprises metal oxide and/or partly oxidised metal which is/arereactable with molten aluminium to form a surface layer containingalumina, aluminium and metal derived from the metal oxide and/or partlyoxidised metal.

[0030] Usually, aluminium-wetted components are completely filled andcovered with aluminium that shields their openly porous or reticulatedceramic structure from exposure to molten electrolyte and/or corrosivegases during use.

[0031] The aluminium-wetted component may be a cathode or a cathodelining or an aluminium pool stabiliser wetted by aluminium before orduring use. The component may be a cell sidewall or a sidewall lining ora wedge-shaped body for joining the surface of a cell bottom to anadjacent sidewall, all wetted by aluminium before use.

[0032] Another aspect of the invention is a cell for the electrowinningof aluminium from alumina dissolved in a fluoride-based electrolyte,comprising one or more aluminium-wettable and/or aluminium-wettedcomponents described above.

[0033] The cell may in particular comprise a cathode or a cathode bodywhose surface is lined with a cathode lining as disclosed above. Thecathode body and the cathode lining may be joined through a bondinglayer, in particular a slurry-applied refractory boride layer asdisclosed in WO01/42168 (de Nora/Duruz) and WO01/42531 (Nguyen/Duruz/deNora). For example, the lined cathode surface is part of a horizontal orinclined cathode bottom, in particular a horizontal cathode bottom linedwith a wedge-like cathode lining forming an aluminium-wettable drainedsloping cathode surface thereon. Alternatively, the cathode body may belocated above a cell bottom that is arranged to collect molten aluminiumproduced on and drained from the cathode lining.

[0034] Further aspects of the invention relate to uses of the abovedescribed material in fields other than the field of aluminiumelectrowinning.

[0035] One further aspect of the invention relates to a compositeceramic-based material which comprises an openly porous or reticulatedceramic structure whose surface during use is exposed to and wetted bymolten aluminium. This structure is made of a ceramic material inert andresistant to molten aluminium and an aluminium-wettable material thatcomprises metal oxide and/or partly oxidised metal selected from partlyoxidised or oxide of copper, nickel, cobalt, manganese and iron andmixtures thereof, which is/are reactable with molten aluminium to form asurface layer containing alumina, aluminium and metal derived from themetal oxide and/or partly oxidised metal.

[0036] Such a material may be used, for instance, for the manufacture ofcomponents or linings of apparatus for treating molten aluminium, inparticular for purifying molten aluminium or separating alloying metalsfrom an aluminium alloy. Further details of such apparatus can be foundin WO00/63630 (Holz/Duruz).

[0037] A yet further aspect of the invention relates to a compositeceramic-metal material which comprises, as before, an openly porous orreticulated ceramic structure which has a surface layer containingalumina, aluminium and another metal. The composite ceramic-metalmaterial is obtainable by exposing to molten aluminium a compositematerial made of a ceramic material inert and resistant to moltenaluminium and an aluminium-wettable material that comprises a metaloxide and/or a partly oxidised metal selected from copper, nickel,cobalt, manganese and iron and mixtures thereof, which is/are reactablewith molten aluminium to form a surface layer containing alumina,aluminium and metal derived from the metal oxide and/or partly oxidisedmetal.

[0038] Such a material may be used for the manufacture ofaluminium-wetted components for applications in high temperatureoxidising or corrosive gases, in particular oxygen and/orfluorine-containing gases, or liquids, such as fluorine-containingliquids or molten metal, in particular molten aluminium.

[0039] In particular, the aluminium-wetted components may be used inapparatus for treating molten aluminium. The components may also be usedat temperatures below the melting point of aluminium as electrodes,heating elements, structural materials, metallurgical crucibles forcontaining molten metals other than aluminium, anodes, furnace fixtures,molds etc. Due to the capacity of the ceramic structure to retain moltenaluminium within its pores and on its surface by capillary effect, thealuminium-wetted components may be used in chemically aggressiveenvironments at temperatures above the melting point of aluminium, forinstance as linings in furnaces, providing the components are notexposed to substantial mechanical wear.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention will be further described with reference to theaccompanying schematic drawings, in which FIGS. 1, 2 and 3 illustratecells of different configurations fitted with aluminium-wettedcomponents of the invention.

DETAILED DESCRIPTION

[0041]FIG. 1 shows an aluminium production cell of drainedconfiguration. The cell comprises non-carbon metal-based anodes 10, forexample as disclosed in WO00/40781 and WO00/40782 (both in the name ofde Nora), which are spaced apart from correspondingly sloped facingcathode surfaces 20, for example as disclosed in WO00/63463 (de Nora),in a fluoride-based molten electrolyte 5.

[0042] The cell bottom 25,25′, for example made of carbon material, iscoveted with aluminium-wetted cathode linings 21,21′ which form drainedaluminium-wetted sloping cathode surfaces 20 according to the invention,different embodiments being shown in the right and the left hand part ofFIG. 1. As shown, the cathode surfaces 20 slope down towards the middleof the cell bottom 25,25′. On the left-hand side of FIG. 1, the cellbottom 25 is horizontal whereas the cathode lining 21′ covering it is awedge with a small angle forming a sloping cathode surface 20 above thehorizontal cell bottom 25. On the right-hand side of FIG. 1, the cellbottom 25′ is at a slope and covered with cathode lining plates (tiles)21 of uniform thickness and which form a sloping cathode surface 20parallel to the sloping cell bottom 25′.

[0043] The cell bottom 25,25′ is only partly covered with the cathodelining 21,21′, leaving a central channel 30 formed by the cell bottom25,25′ and the adjacent cathode linings 21,21′ which are spaced in themiddle of the cell by channel 30. This channel 30 serves to collectproduct molten aluminium 60 from the sloping cathode surfaces 20.

[0044] The cell bottom 25,25′, in particular where it forms part of thealuminium-collection channel is preferably protected with an aluminiumwettable layer 35, for example a slurry-applied refractory boride layeras disclosed in WO01/42168 (de Nora/Duruz) or WO01/42531(Nguyen/Duruz/de Nora). Such a slurry-applied layer 35 is also wetted bymolten aluminium 22 that wets also the bottom of the cathode linings21,21′ providing a continuous and optimal electrical contact.

[0045] As shown in FIG. 1, the cell comprises sidewalls 40, for examplemade of silicon carbide, which are protected with an aluminium-wettedsidewall lining 41 according to the invention. The sidewall lining 41 iscompletely filled with molten aluminium retained in its pores bycapillary effect. The sidewall lining 41 extends vertically from thecell bottom 25,25′ to above the surface of the molten electrolyte 5, andcompletely shields the sidewalls 40 from molten electrolyte 5.

[0046] The aluminium-wetted sidewall lining 41 and cathode linings21,21′ are joined through generally wedge-shaped aluminium-filled bodies51 according to the invention located on the periphery of cell bottom25,25′.

[0047] Thus, all the structural elements except anodes 10 are completelyshielded from the molten electrolyte 5 by molten aluminium retained inand on the aluminium-wetted components according to the invention, or bythe layer of molten aluminium 60 collected in channel 30. Such a cellconfiguration utilising these cell materials permits use of theelectrolyte 5 which is entirely in a molten state, i.e. without frozenelectrolyte ledges along the sidewalls 40 and without a frozenelectrolyte crust at the surface of the electrolyte 5.

[0048]FIG. 2, where the same reference numerals are used to designatethe same elements, illustrates inventive cell components in another cellaccording to the invention.

[0049] The cell shown in FIG. 2 has a horizontal cell bottom 25 which iscovered with an aluminium-wetted cathode lining 21 according to theinvention of uniform width forming a horizontal drained cathode surface20. The sidewalls 40 of the cell are covered with an aluminium-wettedwedge-shaped sidewall lining 41′ that extends from the periphery of thecell bottom 25 to above the surface of the molten electrolyte 5.

[0050] The cell bottom 25 comprises in the middle of the cell, a channel30 for collecting product aluminium 60 drained from the adjacentaluminium-wettable cathode surfaces 20.

[0051] The aluminium collection channel 30 is preferably coated with aslurry-applied refractory boride layer 35 as described above. Theslurry-applied layer 35 is wetted by molten aluminium 22 that wets alsothe bottom of the aluminium-wetted cathode lining 21.

[0052] Similarly to the cell shown in FIG. 1, all the internalstructural elements except anodes 10 are completely shielded from themolten electrolyte 5 by molten aluminium retained in and on thealuminium-wetted components according to the invention or by the layerof molten aluminium 60 collected in channel 30.

[0053] To prevent the electrolyte 5 from freezing along the sidewalllining 41′ and on the surface of the electrolyte 5, the cell isthermally well insulated. As shown in FIG. 2, the cell is fitted with aninsulating cover 45 above the molten electrolyte 5. Further details ofsuitable covers are disclosed in WO01/31086 (de Nora/Duruz).

[0054] The anodes 10 are preferably made of electrolyte resistant inertmetal-based material. Suitable metal-based anode materials include ironand nickel based alloys which may be heat-treated in an oxidisingatmosphere as disclosed in WO00/06802, WO00/06803 (both in the name ofDuruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), WO01/42535 (Duruz/deNora), WO01/42534 (de Nora/Duruz) and WO01/42536 (Duruz/Nguyen/de Nora).Further oxygen-evolving anode materials are disclosed in WO99/36593,WO99/36594, WO00/06801, WO00/06805, WO00/40783 (all in the name of deNora/Duruz), WO00/06800 (Duruz/de Nora), WO99/36591 and WO99/36592 (bothin the name of de Nora).

[0055] To reduce the dissolution of the anodes 10 in the electrolyte,the cell may be operated with an electrolyte 5 at reduced temperature,typically from about 8300 to 930° C., preferably from 8500 to 910° C.Operating with an electrolyte at reduced temperature reduces thesolubility of oxides, in particular of alumina. Therefore, it isadvantageous to enhance alumina dissolution in the electrolyte 5.

[0056] Enhanced alumina dissolution may be achieved by utilising analumina feed device which sprays and distributes alumina particles overa large area of the surface of the molten electrolyte 5. Suitablealumina feed devices are disclosed in greater detail in WO00/63464 (deNora/Berclaz). Furthermore, the cell may comprise means (not shown) topromote circulation of the electrolyte 5 from and to the anode-cathodegap to enhance alumina dissolution in the electrolyte 5 and to maintainin permanence a high concentration of dissolved alumina close to theactive surfaces of anodes 10, for example as disclosed in WO00/40781 (deNora).

[0057] During operation of the cells shown in FIGS. 1 and 2, aluminadissolved in the electrolyte is electrolysed to produce oxygen on theanodes 10 and aluminium 60 on the drained cathode surfaces 20. Theproduct aluminium 60 drains from the cathode surfaces 20 into thecollection channel 30 from where it can be tapped or evacuated into analuminium reservoir (not shown), for example as disclosed in WO00/63463(de Nora).

[0058]FIG. 3 where the same reference numerals are used to designate thesame elements, illustrates a retrofitted cell utilising aluminium-wettedcomponents according to the invention and conventional consumable carbonanodes 10′.

[0059] The cell bottom 25 is horizontal and protected from wear with analuminium-wetted cathode lining 21 according to the invention forming adrained cathode surface 20. The cell sidewalls 40 are covered with asidewall lining 41 according to the invention, extending from the cellbottom to above the surface of the molten electrolyte 5. Thealuminium-wetted sidewall lining 41 and the aluminium-wetted cathodelinings 21 are joined through generally wedge-shaped bodies 51 accordingto the invention.

[0060] The cell bottom 25 is covered with a slurry-applied refractoryboride layer 35 wetted by molten aluminium 22 that wets also the bottomof aluminium-wetted cathode lining 21.

[0061] The cell bottom 25 comprises in the middle of the cell, a channel30 for collecting product aluminium 60 drained from the adjacentaluminium-wettable cathode surfaces 20.

[0062] Unlike the cell shown in FIGS. 1 and 2, the cell shown in FIG. 3operates with a frozen electrolyte crust 70 and ledge 71.

[0063] During operation of the cell shown in FIG. 3, alumina isdissolved into the electrolyte 5 and electrolysed between the carbonanodes 10′ and the drained cathode surface 20 to produce CO₂ at thecarbon anodes 10′ and aluminium which is drained into channel 30.

[0064] In a variation, a retrofitted cell without an aluminiumcollection groove may operate with a shallow aluminium cathodic poolwith little motion of molten aluminium in the shallow cathodic pool.Consequently, the inter-electrode distance may also be reduced whichleads to a reduction of the cell voltage and energy savings.Furthermore, compared to conventional deep pool cells, a smaller amountof molten aluminium is needed to operate the cell which substantiallyreduces the costs involved with immobilising large aluminium stocks inaluminium production plants.

[0065] Nevertheless, these aluminium-wetted cathode linings can also beused in deep pool cells operating with a frozen electrolyte ledge and/oran electrolyte crust above the molten electrolyte. Furthermore, one ormore large aluminium-wetted conductive plates according to the inventionmade from a low density openly porous or reticulated ceramic structuremay be put into the aluminium pool so that the plates float at thesurface of the aluminium pool to restrain aluminium motion and stabilisethe aluminium pool. Thus, use of stabiliser plates in a deep aluminiumpool permits a reduction of the inter-electrode distance.

[0066] In further variations of the above cells only one or some of theabove described cell components according to the invention, i.e. cathodelining 21,21′, sidewall lining 41,41′, wedge-shaped bodies 51 andstabiliser plates, may be used in an aluminium production cell, indifferent combinations.

[0067] The invention will be further described in the followingexamples.

EXAMPLE 1

[0068] An openly porous alumina structure (10 pores per inch which isequivalent to about 4 pores per centimetre) was renderedaluminium-wettable by coating it with two slurry-applied layers ofdifferent composition.

[0069] The first slurry of the first layer was made of 60 weight %particulate needle-shaped surface-oxidised TiB₂ (−325 mesh) having aTiO₂ surface oxide film, 3.3 weight % aluminium-wetting agent in theform of particulate Fe₂O₃ (−325 mesh) and 3.3 weight % TiO₂ powder (−325mesh) in 33 weight % colloidal Al₂O₃ (NYACOL® Al-20, a milky liquid witha colloidal particle size of about 40 to 60 nanometer). When this slurryis heat treated, the colloidal alumina reacts with a TiO₂ surface oxideand the TiO₂ powder to form a mixed oxide matrix of Al₂O₃ and TiO₂throughout the coating, this matrix containing and bonding the TiB₂particles and the Fe₂O₃ particles.

[0070] The second slurry was made of 33 weight % of partly oxidisedcopper particles, 37 weight % of a first grade of colloidal alumina(NYACOL® Al-20) and 30 weight % of a second grade of colloidal alumina(CONDEA® 10/2 Sol, a clear, opalescent liquid with a colloidal particlesize of about 10 to 30 nanometer).

[0071] An aluminium-wettable coating was applied onto the porous aluminastructure by dipping this structure into the first slurry followed bydrying for 4 hours at 40° C. and dipping it into the second slurryfollowed by drying for 15 hours are 40° C. The coated alumina structurewas then heat treated for 3 hours in air at 700° C. to consolidate thecoating.

[0072] The resulting structure is aluminium-wettable and is suitable tobe wetted by aluminium before use or it can be wetted in-situ when usedas a cathode.

[0073] The aluminium-wettable porous structure was wetted with aluminiumby dipping it in molten aluminium at 850° C. After 20 hours the wettedporous structure was extracted from the molten aluminium and allowed tocool down to room temperature.

[0074] Examination of the aluminium-wetted porous structure showed thatit was completely filled with aluminium retained in the pores by thewettability of the structure and the capillary effect, and covered overthe outer surface with aluminium.

[0075] The electrical resistivity of the aluminium-wetted structure wasof the order of the resistivity of metal aluminium (2.65 μΩ.cm), whereasbefore wetting the structure had a resistivity of 35 to 45 kΩ.cm.

[0076] Such a wetted alumina structure can be used for variousapplications in an aluminium electrowinning cell, in particular as acathode or cathode lining, a cell sidewall or a sidewall lining, or as anon current carrying component of the cell bottom which is exposed tomolten aluminium and/or electrolyte.

EXAMPLE 2

[0077] An aluminium-wettable ceramic structure was made of a mixture ofmaterial inert and resistant to molten aluminium, i.e. alumina andtitania, and aluminium-wettable material, i.e. copper oxide. The ceramicstructure was prepared by coating a polyurethane foam with a slurry ofceramic particles followed by a heat treatment.

[0078] The slurry of ceramic material consisted of a suspension of 40 gparticulate Al₂O₃ with an average particle size of 10 to 20 micron, 2.5g of particulate CuO with a particle size of less than about 45 micron,2.5 g of particulate TiO₂ with a particle size of less than about 45micron in a colloidal alumina carrier consisting of 93 g deionised waterand 6.6 g colloidal alumina particles with a colloidal particle size ofabout 10 to 30 nanometer.

[0079] A polyurethane foam having 10 to 20 pores per inch (equivalent toabout 4 to 8 pores per centimetre) was dipped into the slurry and driedin air at 400 to 50° C. for 20 to 30 minutes. The dipping was repeatedthree times.

[0080] After dipping, the foam was dried in air at 50° C. for 4 to 5hours. The foam contained about 0.3 to 0.5 g/cm³ of the dried slurry.The drying was followed by a heat treatment at about 8500 to 1000° C. inair for 4 to 5 hours to eliminate the polyurethane foam and consolidatethe ceramic material formed from the slurry into a self-sustaining foam.This heat treatment was followed by an aluminisation treatment byimmersion in molten aluminium for 2 hours in molten aluminium at 850° C.

[0081] The aluminised foam was extracted from the molten aluminium,allowed to cool to room temperature and cut perpendicular to a surface.

[0082] Examination of the aluminised foam showed that the polyurethanefoam had disappeared. The TiO₂ had reacted with Al₂O₃ in the ceramicfoam to form a titanium-aluminium mixed oxide matrix. CuO present at thesurface of the ceramic foam had reacted with molten aluminium to producean aluminium-wetted surface layer of Al₂O₃ and an alloy of copper andaluminium. The pores of the ceramic foam were completely filled withmolten aluminium.

[0083] In a variation, the heat treatment step and the aluminisationstep are carried out simultaneously as a single step. In a furthervariation, the copper oxide of the ceramic structure is replaced partlyor completely with iron oxide and/or nickel oxide.

EXAMPLE 3

[0084] An aluminium-wettable openly porous ceramic structure as inExample 1 was tested as cathodic material for aluminium production.

[0085] The aluminium-wettable ceramic structure was placed on the bottomof a graphite receptacle having an inner diameter of 85 mm. Thestructure was covered with 120 g aluminium. The receptacle and itscontent was heated at a rate of 120° C./hour. At a temperature of 700°C., the aluminium had formed an aluminium pool on which the ceramicstructure was floating. The temperature was further increased to about850° C. and then maintained for 4 hours so that the molten aluminiumcompletely aluminised and wet the ceramic structure.

[0086] After aluminisation, an amount of 1.5 kg electrolytic molten bathconsisting of 68 weight % cryolite, 28 weight % aluminium fluoride and 4weight % dissolved alumina was poured into the receptacle on top of thealuminium pool and aluminium-wetted ceramic structure. A carbon anodewas dipped into the electrolyte to face the floating ceramic structurewhich formed both an aluminium pool stabiliser and a cathode surface. Anelectrolysis current was passed between the anode and the graphitereceptacle at a current density of about 0.8 A/cm² at the anode. Aconstant cell voltage of about 4 to 4.2 volt was measured throughoutelectrolysis.

[0087] After 10 hours, electrolysis was interrupted and the floatingaluminium-wetted ceramic structure extracted from the graphitereceptacle.

[0088] The ceramic structure was allowed to cool down to roomtemperature and cut perpendicular to one of its surfaces. Examination ofthe ceramic structure showed that it was still completely wetted by andfilled with molten aluminium. The ceramic structure itself had remainedunchanged demonstrating its stability and suitability as cathodematerial.

EXAMPLE 4

[0089] An openly porous silicon carbide structure (30 pores per inchwhich is equivalent to about 12 pores per centimetre) was renderedaluminium-wettable by coating it with a slurry-applied layer.

[0090] The slurry consisted of 75 g surface oxidised iron particles(−325 mesh), 75 g Silica sol Nyacol 830 (a milky aqueous liquidcontaining 32 weight % colloidal silicon hydroxide that is convertedinto silica upon heat treatment) and 0.35 g of an aqueous solutioncontaining 15% PVA (polyvinyl alcohol) that was used to adjust theviscosity of the slurry.

[0091] The openly porous structure was dipped onto the slurry and thendried for 30 min. at 60° C. The impregnated porous structure contained0.278 g/cm³ of dried slurry including 0.214 g/cM³ surface oxidised ironparticles.

[0092] The resulting structure was aluminium-wettable and suitable to bewetted by aluminium before use or in-situ when used for example as acathode.

[0093] The aluminium-wettable porous structure was wetted with aluminiumby dipping it in molten aluminium at 850° C. After 15 hours the wettedporous structure was extracted from the molten aluminium and allowed tocool down to room temperature.

[0094] Examination of the aluminium-wetted porous structure showed thatit was filled with aluminium retained in the pores by the wettability ofthe structure and the capillary effect, and covered over the outersurface with aluminium. The pores had an aluminium filling ratio thatwas greater than 90 vol %.

[0095] The aluminium-wetted porous structure can be used as cathodicmaterial like in Example 3.

1. An aluminium-wettable component of a cell for the electrowinning ofaluminium from alumina dissolved in a fluoride-based molten electrolyte,said component comprising an openly porous or reticulated ceramicstructure whose surface during use is exposed to and wetted by moltenaluminium, the structure being made of: a ceramic material inert andresistant to molten aluminium, in particular a material comprising atleast one oxide, carbide, nitride or boride selected from: oxides ofaluminium, zirconium, tantalum, titanium, silicon, niobium, magnesiumand calcium and mixtures thereof, as a simple oxide and/or in a mixedoxide; and aluminium nitride, AlON, SiAlON, boron nitride, siliconnitride, silicon carbide, aluminium borides, alkali earth metalzirconates and aluminates and mixtures thereof; and analuminium-wettable material that comprises metal oxide and/or partlyoxidised metal which is/are reactable with molten aluminium to form onthe openly porous or reticulated ceramic structure a surface layercontaining alumina, aluminium and metal derived from said metal oxideand/or partly oxidised metal, the metal of said metal oxide and/orpartly oxidised metal being in particular selected from manganese, iron,cobalt, nickel, copper and zinc and combinations thereof.
 2. Thecomponent of claim 1, wherein the openly porous or reticulated ceramicstructure comprises a coating of the aluminium-wettable material on theinert and resistant ceramic material.
 3. The component of claim 2,wherein the aluminium-wettable coating is a slurry-applied coatingcomprising particles of said reactable metal oxide and/or partlyoxidised metal in a dried colloidal carrier selected from alumina,ceria, lithia, magnesia, silica, thoria, yttria, zirconia, titaniumoxide and zinc oxide, and mixtures and precursors thereof.
 4. Thecomponent of claim 3, wherein the slurry-applied aluminium-wettablecoating further comprises particles of at least one compound selectedfrom metal borides, carbides and nitrides.
 5. The component of claim 4,wherein the slurry-applied aluminium-wettable coating comprises theparticles of said reactable metal oxide and/or partly oxidised metal andparticles of titanium diboride in dried colloidal alumina.
 6. Thecomponent of claim 4 or 5, wherein particles of a metal boride, carbideor nitride are covered with mixed oxides of metal derived from the driedcolloidal carrier and metal derived from the metal boride, carbide ornitride.
 7. The component of claim 6, wherein the slurry-appliedaluminium-wettable coating is obtainable from a slurry that comprisesmetal oxide particles that combine upon heat treatment with a metaloxide derived from the dried colloidal carrier to form mixed oxideswhich are miscible with said mixed oxides covering the particles ofmetal boride, carbide or nitride.
 8. The component of claim 1, whereinthe openly porous ceramic structure is made of a composition whichconsists of a mixture of the inert and resistant ceramic material andthe aluminium-wettable ceramic material.
 9. The component of anypreceding claim, wherein the openly porous ceramic structure is formedon a reinforcing metal skeleton.
 10. The component of any precedingclaim, which comprises an internal insert acting as ballast.
 11. Thecomponent of any preceding claim, which is a cathode or a cathodelining.
 12. The component of any one of claims 1 to 10, which is analuminium pool stabiliser in the form of a plate.
 13. The component ofany one of claims 1 to 10, which is a skeleton of a cell sidewall or asidewall lining, which skeleton can be filled with molten aluminium toform an aluminium-infiltrated cell sidewall or sidewall lining.
 14. Thecomponent of any one of claims 1 to 10, which is a skeleton of awedge-shaped connecting body for joining the surface of a cell bottom toan adjacent sidewall, which skeleton can be filled with molten aluminiumto form an aluminium-infiltrated connecting body.
 15. Analuminium-wetted component of a cell for the electrowinning ofaluminium, said aluminium-wetted component comprising an openly porousor reticulated ceramic structure which has a surface layer containingalumina, aluminium and another metal obtainable by exposing analuminium-wettable component according to any preceding claim to moltenaluminium.
 16. The aluminium-wetted component of claim 15, which isfilled and covered with aluminium that shields the openly porous orreticulated ceramic structure from exposure to molten electrolyte and/orcorrosive gases during use.
 17. The aluminium-wetted component of claim15 or 16, which is a cathode or a cathode lining.
 18. Thealuminium-wetted component of claims 15 to 16, which is an aluminiumpool stabiliser in the form of a plate.
 19. The aluminium-wettedcomponent of claim 16, which is a cell sidewall or a sidewall lining.20. The aluminium-wetted component of claim 16, which is a wedge-shapedbody for joining the surface of a cell bottom to an adjacent sidewall.21. A cell for the electrowinning of aluminium from alumina dissolved ina fluoride-based electrolyte, comprising at least one aluminium-wettablecomponent as defined in any one of claims 1 to 14 and/or at least onealuminium-wetted component as defined in any one of claims 15 to
 20. 22.The cell of claim 21, which comprises a cathode or a cathode lining asdefined in claim 11 or
 18. 23. The cell of claim 22, which comprises acathode body having a surface lined with a plate-like or wedge-likecathode lining.
 24. The cell of claim 23, wherein the cathode body isjoined to the cathode lining through a bonding layer.
 25. The cell ofclaim 24, wherein the lined cathode surface is part of a horizontal orinclined cathode bottom.
 26. The cell of claim 25, wherein the cathodebottom is horizontal and lined with a wedge-like cathode lining formingan aluminium-wettable drained sloping cathode surface thereon.
 27. Thecell of claim 22, 23 or 24, wherein the cathode or cathode lining islocated above a cell bottom that is arranged to collect molten aluminiumproduced on and drained from the cathode or cathode lining.
 28. The cellof any one of claims 22 to 27, comprising a cathode or cathode lining asdefined in claim 9 which is top coated with an aluminium-wettablestart-up layer.
 29. The cell of claim 21 or 22, comprising one or morepool stabilisers as defined in claims 12 or 19 floating on an aluminiumpool contained in the cell.
 30. The cell of any one of claims 21 to 29,which comprises a cell sidewall or a sidewall lining as defined in claim19.
 31. The cell of claim 30, comprising a sidewall lining as defined inclaim 19 that covers a sidewall made of carbon-containing material. 32.The cell of any one of claims 21 to 31, comprising at least onewedge-shaped connecting body as defined in claim 20 joining the cellbottom to an adjacent sidewall.
 33. A composite openly porous orreticulated ceramic structure whose surface is wettable by moltenaluminium, the structure being made of: a ceramic material inert andresistant to molten aluminium, in particular a material comprising atleast one oxide, carbide, nitride or boride selected from: oxides ofaluminium, zirconium, tantalum, titanium, silicon, niobium, magnesiumand calcium and mixtures thereof, as a simple oxide and/or in a mixedoxide; and aluminium nitride, AlON, SiAlON, boron nitride, siliconnitride, silicon carbide, aluminium borides, alkali earth metalzirconates and aluminates and mixtures thereof; and analuminium-wettable material that comprises metal oxide and/or partlyoxidised metal which is/are reactable with molten aluminium to form onthe openly porous or reticulated ceramic structure a surface layercontaining alumina, aluminium and metal derived from said metal oxideand/or partly oxidised metal, the metal of said metal oxide and/orpartly oxidised metal being in particular selected from manganese, iron,cobalt, nickel, copper and zinc and combinations thereof.
 34. Acomposite ceramic-metal material comprising an openly porous orreticulated ceramic structure which has a surface layer containingalumina, aluminium and another metal, said composite ceramic-metalmaterial being obtainable by exposing to molten aluminium a compositeopenly porous or reticulated ceramic structure as defined in claim 33.